1. Field of the Invention
The present invention relates in general to a control apparatus for a vehicular power transmitting system including a transmission portion, and more particularly to techniques for improving a shifting action of the transmission portion as felt by an operator of a vehicle.
2. Discussion of Prior Art
There is known a vehicular power transmitting system including (a) a differential mechanism configured to distribute an output of an engine to a first electric motor and a power transmitting member, (b) a second electric motor connected to a power transmitting path between the power transmitting member and drive wheels of a vehicle, and (c) a an automatic transmission portion which constitutes a part of the power transmitting path. JP-2005-319924 A discloses an example of such a vehicular power transmitting system. The automatic transmission portion of this vehicular power transmitting system is provided with a plurality of hydraulically operated coupling elements, and a control apparatus for the vehicular power transmitting system is arranged to selectively release and engage the coupling elements according to a predetermined shifting boundary line map, for implementing a shifting action of the automatic transmission portion. Usually, the control apparatus stores information indicative of an operable zone of the first electric motor, which is obtained by experimentation while taking account of thermal durability and mechanical durability of the first electric motor. This operable zone is not disclosed in the above-identified publication. The periphery of the operable zone is defined by an operable limit. The control apparatus controls the first electric motor to be operated within the operable zone such that there exists a given amount of margin with respect to the operable limit.
The hydraulically operated coupling elements of the automatic transmission portion are operated with a certain time delay with respect to a moment of generation of a control signal generated from the control apparatus for the vehicular power transmitting system. The amount of the operational time delay of the coupling elements varies as a function of a temperature of a working oil or fluid used for operating the coupling elements. For example, the coupling elements have a larger amount of the operational time delay, when the working fluid temperature is extremely low, than when the working fluid temperature is held at a steady-state value after a warm-up operation of the automatic transmission portion. Accordingly, a shifting response of the automatic transmission portion is reduced (deteriorated) as the working fluid temperature is lowered. Although the first electric motor is operated within the operable zone with a given amount of margin with respect to the above-indicated operable limit while the working fluid temperature has a steady-state value, there is a possibility that an operating point of the first electric motor reaches the operable limit during a time period from the moment of generation of the control signal to initiate a shifting action of the automatic transmission portion to the moment of completion of the shifting action, and the output of the first electric motor is limited by the operable limit, when the shifting response of the automatic transmission portion is low, namely, when the coupling elements have a relatively large amount of the operational time delay with respect to the moment of generation of the control signal. This possibility is not addressed in the prior art. The output limitation of the first electric motor influences a drive force transmitted to the drive wheels, whereby the shifting action of the automatic transmission portion as felt by the vehicle operator is deteriorated. However, the control apparatus for the vehicular power transmitting system disclosed in the above-identified publication is not configured to provide any remedy for reducing or minimizing the deterioration of the shifting action of the automatic transmission portion as felt by the vehicle operator when the shifting response is low.